We have been using rat fat cells in an attempt to elucidate the mechanism of action of insulin. By exposing whole cells to insulin, then preparing isolated plasma membrane vesicles, we can measure glucose transport in a cell-free system which does not metabolize glucose. Membranes prepared from insulin-exposed cells show accelerated glucose transport. Pre-treatment of whole cells with energy poisons blocks the action of insulin without affecting insulin binding. We propose to explore more fully the relation between high energy phosphate bonds and the ability of the cell to respond to insulin with accelerated glucose transport. We have also noted distinctive changes in cell surface glycoproteins in animals with experimentally induced diabetes. These changes are reversed by insulin treatment of the animals. We propose to quantitate the changes in carbohydrate chains of cell surface glycoproteins prepared from liver cells of normal and diabetic rats.